1. Field of the Invention
This invention relates to methods for manufacturing tubes of carbon steel, stainless steel, copper alloy, aluminum alloy and other metals filled with a powdery and/or granular substance.
The powdery and granular substances are powders, granules or their mixtures, such as welding fluxes, oxide-based superconductors and steelmaking additives. This invention is used in the manufacture of wires containing welding fluxes, wires containing oxide-based superconductors and other tubes containing a powdery and/or granular substance.
2. Description of the Prior Art
Seamless wire containing welding flux is an example of tubes filled with a powdery and/or granular substance. The seamless wire is made by slitting steel strip into desired widths and gradually forming the slit strip formed into a U-shape, and then into an O-shape, using a series of forming rolls. Halfway in the forming process, a flux is fed from a feeder into the bottom of the U-shaped strip through an opening extending along the length thereof. When the U-shaped strip is formed into an O-shape, the meeting edges of the strip are welded together to close the opening. Then, the diameter of the welded shell is reduced. After being annealed as required, the tube filled with the flux is drawn into a wire of the desired diameter and coiled into the desired product form.
Low-frequency welding, high-frequency induction welding and high-frequency resistance welding are extensively used in the manufacture of tubes filled with a powdery and/or granular substance. In any of these welding methods, the edges of the strip fringing the opening are heated to the melting temperature by a low-frequency or high-frequency current and then pressed until they meet and form the weld by a pair of squeeze rolls.
The welded tubes filled with the flux may break in the subsequent process in which their diameter is reduced by rolling and drawing. This break is considered to result from the following cause. When welding is performed, some of the oxide or silicate in the flux adheres to the fringing edges of the opening in the formed tube. At the welding point, air flows outside from the formed tube through the opening as a result of the expansion caused by the collision of the air carried in by the approaching formed tube and the air flowing backward from the size-reducing point and by the heat of welding. The stream of air thus created blows up part of the flux, which adheres to the fringing edges of the opening in the formed tube. Some of the flux jumping up under the influence of the vibration of the approaching formed tube too adheres to the same area. Because of a magnetic field built up by the welding current at the welding point, in addition, the fringing edges of the formed tube serve as a magnetic pole. Therefore, the magnetic force of the fringing edges attracts the ferromagnetic ingredients of the flux. At this time, the ferromagnetic ingredients take some nonmagnetic components with them to the fringing edges. The flux thus adheres to the fringing edges fuses into the weld to form nonmetallic inclusions detrimental to the weld. This welding defect leads to cracking or breaking in the subsequent size reduction process.
The "Method and Apparatus for Manufacturing Filler Wire" disclosed in Japanese Provisional Patent Publication No. 234795 of 1985 offers a solution for this type of problem. This technology prevents the blow-up of the powder by drawing in the stream of air created in the formed tube upstream of the welding or roll-press zone. The "Method of Manufacturing Filler Wire" disclosed in the Japanese Provisional Patent Publication No. 234792 of 1985 offers another solution. This technology forms a lower layer a ferromagnetic or ferrite-based substance and an upper layer of a nonmagnetic substance so that the latter inhibits the attraction of the former to the fringing edges of the opening. The "Composite Welding Wire" disclosed in the Japanese Provisional Patent Publication No. 234794 of 1985 discloses a still another solution. This technology fills a substantially nonmagnetic powder whose relative magnetic permeability is not higher than 1.10 to prevent the powder from getting magnetically attracted to the fringing edges of the opening. The "Method of Manufacturing Tubes Filled with Powders" disclosed in the Japanese Provisional Patent Publication No. 109040 of 1979 relates to a technology that does not fill a tube 100% with a powder in order to leave such a space or distance between the weld and the surface of the powder as is large enough to keep the blown-up powder away from the fringing edges of the opening.
The "Method of Manufacturing Wires Filled with Powders" disclosed in the Japanese Provisional Patent Publication No. 125436 of 1977 exhibits another technology that granulates all or part of those ingredients of a powder whose size is finer than 250 mesh with a suitable binder. This technology is intended for the improvement of feedability through granulation, rather than the prevention of the presence of nonmetallic inclusions in the weld of a powder-filled tube. But the increased powder particle size achieved by granulation seems to have an effect on the prevention of the blow-up of the powder exposed to a stream of air.
Even after the introduction of the aforementioned weld improving technologies, however, breakage have continued to occur in the tube-size reducing process, entailing a drop in working efficiency and project yield. Breakage has occurred more frequently as the amount of drawing and size reduction increased. The tendency has been particularly pronounced when the diameter Of the final product was 1.6 mm or under.
The technology to suck the stream of air generated in the tube sometimes produces an adverse effect as a new air stream caused by the suction blows up, rather than settles, the powder inside. The technology to spread a layer of a nonmagnetic material on top of a layer of a ferromagnetic or ferrite-based material and the technology to fill a substantially nonmagnetic powder cannot prevent the nonmagnetic powder from getting blown up or jumping up under the influence of the vibration of the tube. Especially when the above materials are spread in two layers, one on top of the other, the nonmagnetic powder in the upper layer springs up because of the vibration of the lower layer caused by the alternating flux passing therethrough. The amount of the powder fed into a tube must be large enough to fill a certain percentage of the cross-sectional area thereof. Therefore, the space left between the weld and the surface of the powder is not always allowed to be large enough to prevent the powder from getting carried to the fringing edges of the opening by the stream of air, the vibration of the tube and the magnetic force of the fringing edges of the opening. Even when finer ingredients of a powder are granulated into larger particles, the ferromagnetic ingredients in the granulated material are attracted to the fringing edges of the opening. Together with the ferromagnetic ingredients, such nonmagnetic ingredients as might form nonmetallic inclusions can sometimes adhere to the fringing edges of the opening.
During the manufacture of tubes filled with a powdery and/or granular substance, large sagging beads are often formed on the inside of the weld. While the beads formed on the outside can be removed by scarfing, but those on the inside of the tubes filled with a powdery and/or granular substance cannot be removed. Tubes carrying such large internal beads have often cracked in the vicinity of the weld in the size-reduction process or broken in the drawing process.
The "Method of Manufacturing Welding Wires Filled with Flux" disclosed in the Japanese Provisional Patent Publication No. 240199 of 1987 offered a solution for the problems just described. This technology uses a welded tube with such internal beads whose width and height, together with the angle formed between their root and the inner surface of the tube, are kept within certain limits. With a powder of flux filled by the vibrating method according to the Japanese Patent Publication No. 30937 of 1970 or other proper method, the tube is drawn to a wire of the desired diameter. If their width etc. are kept within certain limits, the internal beads are not pressed into the tube wall even when they are deformed in the drawing process. Such beads also prevent the occurrence of notches at their root and cracks in the vicinity of the weld.
The inventors are aware that internal beads of satisfactory shape and size can be obtained even when welding is performed with an inverted-V groove so long as the angle of the groove and the amount of heat input are kept within proper limits. The groove angle varies with the forming schedule of the tube. As the edges of the formed tube are butted and continuously welded together, it is practically impossible to measure the groove angle of tubes with small wall thickness and small diameter. Their groove angle must be estimated on the basis of their forming schedule. Even the estimation of the groove angle is difficult with the tubes filled with a powdery and/or granular substance covered by this invention as their diameter and wall thickness at the butt welding point are very small (e.g., 21.7 mm in outside diameter and 2.2 mm in wall thickness). The proper heat input that forms satisfactory internal beads varies with the groove angle as well. Therefore, the conditions of heat input derived from the estimated groove angle have involved such considerable errors that the shape and size of internal beads have varied greatly.
The Japanese Provisional Patent Publication No. 240199 of 1987 shows an example of welding with an inverted-V groove (only one example with a groove angle of 15 degrees). But it neither discloses nor suggests the welding conditions, including the groove profile, that will provide satisfactory internal beads.